1. Field of the Invention
This invention relates to a silicon carbide semiconductor device and particularly to a silicon carbide semiconductor device having a gate insulating film and a method for manufacturing the same.
2. Description of the Background Art
In an Si (silicon) MOSFET (Metal Oxide Semiconductor Field Effect Transistor) representing a widely used power semiconductor device, a main factor determining a breakdown voltage is an upper limit of electric field intensity which a drift layer forming a breakdown voltage holding region can withstand. A drift layer made of Si may break down at a portion where electric field not lower than approximately 0.3 MV/cm is applied. Therefore, it is necessary to suppress electric field intensity to be less than a prescribed value in the entire drift layer of a MOSFET. The simplest method is to lower an impurity concentration in a drift layer. This method, however, is disadvantageous in high ON resistance of a MOSFET. Namely, there is trade-off relation between ON resistance and a breakdown voltage.
Japanese Patent Laying-Open No. 9-191109 describes trade-off relation between ON resistance and a breakdown voltage in connection with a typical Si MOSFET, taking into consideration a theoretical limit obtained from a physical property value of Si. Then, in order to overcome this trade-off, it has disclosed addition of a lower p-type embedded layer and an upper p-type embedded layer in an n base layer on an n-type substrate on a drain electrode. The lower p-type embedded layer and the upper embedded layer divide the n base layer into a lower portion, a middle portion, and an upper portion equal to one another in thickness. According to this document, an equally divided voltage is applied to the three portions and maximum electric field of each portion is kept at limit electric field intensity or lower.
As a method for greatly improving the trade-off described above, use of SiC (silicon carbide) instead of Si has actively been discussed in recent years. Unlike Si, SiC is a material which can sufficiently withstand even electric field intensity not lower than 0.4 MV/cm.
In a case where such high electric field may be applied, breakdown due to electric field concentration at a specific position in a MOSFET structure gives rise to a problem. For example, in a case of a trench-structure MOSFET, a breakdown phenomenon of a gate insulating film due to electric field concentration in the gate insulating film at a bottom portion, in particular a corner portion, of a trench is a main factor determining a breakdown voltage. Thus, a factor determining a breakdown voltage is different between an Si semiconductor device and an SiC semiconductor device. Therefore, if the technique in the document above which is considered to be premised on use of Si is simply applied for improvement of a breakdown voltage of an SiC semiconductor device, improvement in breakdown voltage by making full use of advantages in terms of physical properties of SiC cannot be achieved.